Not applicable.
This invention relates to optical attenuators, and particularly to variably controlled, fiber-coupled, microelectromechanically actuated optical attenuators.
Many types of optical attenuators have been developed since the onset of optical telecommunication. As commonly known, optical attenuators are related to optical switches wherein the attenuators afford a stepwise or entirely variable control of the intensity reduction of the incident optical beam.
Fiber-coupled optical attenuators can be made by aligning and spacing two angle-cleaved optical fibers so that the two fibers define a substantially common optical axis, with the spacing being only as large, with the view to limiting the diffraction loss, as to insert a blocking vane into the gap between the two xe2x80x9cbutt-coupledxe2x80x9d fibers. As exemplified e.g. by U.S. Pat. No. 6,222,656 or U.S. Pat. No. 6,275,320, the vane may be opaque and may function by blocking partially or entirely the incident beam.
Also known are optical attenuators with a rotary variable-transmissivity attenuating element. Attenuation is effected by positioning the element in the path of the incident light beam such that predetermined portions of the element of different transmissivity face the incident beam.
Advances in semiconductor and thin film technology have enabled the development of micro-electro-mechanical system (MEMS) structures. MEMS structures, available in size on the order from a few hundred microns to a few millimeters, are typically capable of motion or applying force. They are also used in a variety of optical applications that include light switches and attenuators. The latter case is exemplified by a recent U.S. Pat. No. 6,275,320 to Dhuler (MEMS Variable Optical Attenuator).
In a MEMS environment, the application of rotary vanes is not very practical and linearly-movable vanes are the typical choice. In this context, it has been found that the use of an absorbing or reflecting vane can result in a large PDL (polarization dependent loss) at attenuations above 5 dB. For example, in an attenuator with a gold-coated vane, adjusted for an attenuation of 20 dB, the PDL is likely to exceed 1 dB. An attenuator described in the Tutorial xe2x80x9cSilicon Micromachines in Lightwave Networks,xe2x80x9d by David Bishop, Optical Fiber Communication Conference (OFC ""99), Feb. 21-26, 1999, is an example of such design. This loss arises because absorbing and reflecting materials exhibit conduction currents in response to incident light, and the shape of the vane allows higher current in the direction parallel to the edge of the vane than in the direction perpendicular to the vane.
It is an object of the invention to eliminate or reduce at least some of the problems associated with the above-discussed prior art, and to provide a small-size fiber-coupled optical attenuator offering at least a reduced PDL compared to analogous prior art devices.
In one aspect, the present invention provides an optical attenuator for optically attenuating an optical beam passing from one optical fiber end to another optical fiber end along an optical axis, the fiber ends spaced from each other by a gap, the attenuator comprising
a substrate supporting the optical fiber ends,
an actuator, and
a transparent or translucent light-diverting element associated with and actuatable by said actuator for movement into and out of the optical beam, the element configured for diverting at least a part of the optical beam off the optical axis when moved into the path of the optical beam.
The element may have at least one surface disposed at a non-perpendicular angle relative to the optical axis for diffracting at least a part of the beam off its axis.
The angle of diffraction may be an acute angle, whereby at least part of the optical power of the beam is shifted, refracted or diverted but not reflected.
Preferably, the attenuator is embodied in a MEMS structure, i.e. the fiber ends, the actuator and the optical light-diverting element (called also xe2x80x9cvanexe2x80x9d), are provided on a microelectronic generally planar substrate.
It is preferable that the vane be of dielectric material e.g. of silicon because of lower induced currents at the boundaries. A dielectric with moderate index of refraction is best because a very high refractive index would result in higher induced currents, and likely in higher PDL.
The refractive index of silicon is about 3.4 at 1550 nm, which is higher than desired (silicon is essentially transparent at wavelengths longer than about 1400 nm). The anti-reflection coating, in addition to suppressing back-reflections, also creates a lower index in the boundary region, reducing the induced currents and the PDL.
The vane should be at least somewhat transparent rather than strongly absorbing, because an absorbing material would melt or bum when operating at high optical power. Even if the vane attains a high temperature below the melting point, the heat could affect the thermal actuator or radiate blackbody radiation into the fibers.